The main olfactory and vomeronasal (accessory olfactory) systems are two of several nasal chemoreceptive systems present in terrestrial vertebrates. They are major sources of direct exteroceptive information to the limbic system and have been implicated in a number of important species-typical behavior such as response to pheromones and prey chemicals. Olfactory and vomeronasal systems differ in several respects including the optimal physical state of stimulating substances. The present proposal addresses the issues of the mechanisms of stimulus access to the respective sensory epithelia, interactions between the two systems, their differential sensitivities to chemical stimuli, and central mechanisms that effect coding and transmission of information from the periphery. Due to the snake's extraordinarily well developed vomeronasal system and the importance of this system in naturally occuring snake behaviors. This animals are excellent models for studying vomeronasal function. Information obtained from past studies on snakes have been usefully applied to research on the mammalian vomeronasal and olfactory systems. The studies in this proposal use biologically relevant odorants (proteins purified from prey) and physiological techniques to sort out the mechanisms of activation of the garter snake vomeronasal system and its relationship to the main olfactory system. The following questions are addressed: 1. Is the mode of delivery of odorants critical to the segregation of chemicals stimulating the olfactory and vomeronasal systems? 2. Does the electrovomeronasogram, the vomeronasal equivalent of the electroolfactogram. represent evoked summed unit (neural) activity? 3. Does activity in the main olfactory system affect response of the vomeronasal system to chemical stimuli? 4. Are there specific receptors for different prey chemicals and are they differentially expressed in animals with different prey preferences? 5. Does the response to amino acids previously observed in the vomeronasal system represent a useful model for investigating receptor-ligand interactions? 6. What is the basis of central modification of receptor-initiated response to chemical stimuli? Results from these studies will provide information on how olfactory and vomeronasal systems are differentially activated by chemical stimuli, from the perspective of odorant access as well as the types of stimuli to which they are preferentially sensitive. In addition, we will obtain information on the manner by which one system may modulate the sensitivity of the other.